This invention relates generally to a staged catalytic process for reducing the ammonia concentration in the gas produced by a gasifier, particularly an oxygen-blown coal gasifier in an integrated gasification combined cycle power plant equipped with high temperature desulfurization.
In integrated gasification combined cycle (IGCC) power plants, low Btu fuel gas produced by a gasifier is burned and expanded through a gas turbine, and the exhaust heat from the gas turbine is used to generate steam for a steam turbine. The low Btu fuel gas can be produced by gasifying coal, biomass, municipal solid waste, wood chips, heavy residual oil, petroleum coke, refinery wastes and other materials. As used herein, the term xe2x80x9cfuel gasxe2x80x9d refers to gas produced by any such gasification process. IGCC systems are attractive because of their high efficiency and because they can use relatively abundant and/or inexpensive energy sources.
Since the fuel gas produced by gasification typically contains high levels of hydrogen sulfide (H2S), a sulfur removal system must be employed. Currently, both low temperature and high temperature desulfurization schemes are used. Hot gas clean up (HGCU) is a high temperature sulfur removal scheme which has several advantages over low temperature schemes, most notably increased system efficiency and decreased cost. HGCU reduces the sulfur in the fuel gas to less than 50 ppmv H2S and is typically carried out in the range of approximately 800-1200xc2x0 F. This temperature regime is near optimal for desulfurization because at temperatures below about 800xc2x0 F. the overall power plant efficiency decreases, while at temperatures above about 1200xc2x0 F. the efficiency and stability of the desulfurization sorbents decrease. However, high temperature fuel gas tends to have a high ammonia content, about 1000-2000 ppmv. This high ammonia content results in high NOx emissions when the fuel gas is burned. Thus, the ammonia content of the high temperature fuel gas must be decreased to reduce NOx emissions.
One way to reduce the ammonia content of the fuel gas is to promote ammonia decomposition. However, known catalysts that are active for ammonia decomposition in the range of 800-1200xc2x0 F. are easily poisoned by as low as a few parts per million of H2S. At temperatures where sulfur poisoning is less of a problem (about 1400xc2x0 F.), these catalysts have poor mechanical/chemical stability, i.e., loss of surface area because of sintering. Similarly, catalysts that are sulfur resistant and mechanically stable at 1400xc2x0 F. tend not to be active enough towards ammonia decomposition at lower temperatures near 1000xc2x0 F. Hence, operation of an ammonia decomposition catalyst at the same temperature as a high temperature desulfurization system may not be easily implementable.
Accordingly, there is a need for a process and apparatus for reducing the ammonia concentration of high temperature fuel gas which can fit into the constraints of an IGCC power plant having high temperature sulfur removal.
The above-mentioned needs are met by the present invention in which the ammonia content of fuel gas is reduced through ammonia decomposition. This is accomplished with a staged catalytic process in which the ammonia decomposition reaction is facilitated by first promoting CO methanation and water-gas-shift.
Specifically, the present invention provides a power generation system comprising a gasification unit, a hot gas desulfurization system, a particulate removal system, and a gas turbine,. A catalytic reactor is arranged between the hot gas desulfurization system and the gas turbine. The catalytic reactor contains a plurality of catalysts which collectively promote water-gas-shift, methanation of CO, and ammonia decomposition. Preferably, a second catalytic reactor is provided in parallel with the first reactor so that the two reactors can alternately receive fuel gas from the desulfurization system. A coolant injection port or heat exchanger is formed in the gas line connecting said the catalytic reactor and the gas turbine to cool the fuel gas.
Each catalytic reactor may be a three stage device that includes a first catalyst which promotes water-gas-shift, a second catalyst which promotes methanation of CO, and a third catalyst which promotes ammonia decomposition. The catalytic reactor can comprise a single vessel containing all of the catalysts, or it can comprise a first vessel containing the first catalyst, a second vessel containing the second catalyst, and a third vessel containing the third catalyst. The three vessels are connected in order between the desulfurization system and the gas turbine. The power generation system can further include heat exchangers disposed in the first and/or second vessels. The heat exchangers can be connected to the injection port or to the steam turbine.
Alternatively, the catalytic reactor may be a two stage device that includes a first catalyst which promotes water-gas-shift and methanation of CO, and a second catalyst which promotes ammonia decomposition. This catalytic reactor can comprise a first vessel containing the first catalyst and a second vessel containing the second catalyst. The vessels are connected in order between the desulfurization system and the gas turbine. If using heat exchange, a heat exchanger would be disposed in the first vessel.
The vessels can be filled with catalysts in either a pelletized form or a fluidized bed. Alternatively, the catalytic reactor can comprise two or three honeycomb structures. Each honeycomb structure would be coated with the appropriate catalyst.
An advantage of the staged catalytic operation of the present invention is that it allows catalysts which individually are restricted to a narrow range of operating conditions to be used in sequence to accomplish the goal of reducing ammonia content that is not feasible by any single stage.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.